CNC turning machine

ABSTRACT

A turning machine is characterized by a new and improved control and mechanism for positioning the turning tool on the head. A CNC control issues command signals for selectively positioning the tool on the head. These signals are developed from an encoder which provides the instantaneous position of the rotating part being turned and from a program in the CNC containing information about the part. A closed loop control system receives these command signals and converts same into a control current for a linear motor which is the prime mover controlling the positioning of the tool on the head. The linear motor operates a carriage on the head and the cutting tool mounts on the carriage opposite the connection of the linear motor to the carriage. The carriage is a hollow bar guided on the head by sets of rollers which are cooperatively arranged to provide yieldably forceful constraint of the bar. The rolling action of the bar on the head provides a low friction, low inertia construction enabling rapid response to CNC commands. The head also contains various sensors providing position and velocity feedback information for use by the closed loop control. The closed loop control contains various circuit components organized and arranged to provide fast and faithful response to command signals. The machine has the ability to accurately turn complex parts where position information is being rapidly updated, often in the tens of kilohertz frequency range. Different parts can be turned by merely changing the CNC program. 
     The questions raised in reexamination request No. 90/001,657, filed Dec. 1, 1988, have been considered and the results thereof are reflected in this reissue patent which constitutes the reexamination certificate required by 35 U.S.C. 307 as provided in 37 CFR 1.570(e).

.Iadd.This is a continuation of co-pending application Ser. No.07/329,885 filed on Mar. 28, 1989, abandoned. .Iaddend.

BACKGROUND AND SUMMARY OF THE INVENTION

This invention relates generally to turning machines and specifically toa new and unique CNC turning machine which is adapted for complexturning of parts such as pistons.

An example of a known turning machine is illustrated in U.S. Pat. No.3,869,947, commonly assigned. In the turning machine disclosed in tharpatent, a part to be turned, for example a piston, is suitably chuckedand rotated about its axis at an appropriate speed. A cutting tool isarranged to make a pass along the rotating part and machine the outerpart surface, i.e. the piston skirt. As the cutting tool is making itsaxial pass along the part, the cutting tool's radial position iscontinuously correlated with the rotation of the part to produce adesired shape. This correlation is achieved by a cam and followersystem. Such a system is capable of imparting both eccentricity andtaper to the part, i.e. complex turning. In other words in the case of apiston, the system is capable of producing either straight or taperedskirts of either circular or elliptical cross section, depending uponthe desired shape.

Such a turning machine is well suited for making large numbers ofidentical parts. However if different shaped parts are to be turned, themachine must be shut down for change-over. When such shut-down occurs,the machine is removed from productive use, and where precision partsare involved, as is usually the case, care must be taken to ensure thatthe new cam and the follower produce the desired precision. The amountof set-up time for the new cam also adds to the total machine downtime.

Turning operations of the type conducted on piston skirts typicallyinvolve relatively high rotational speeds for the pistons. In the camand follower type system described above, the dynamics of the machineand mechanism can limit the ability of the follower to track the cam.Hence minor changes in the cam surface shape may be difficult to follow,and there is necessarily a limit to the ultimate precision with whichparts can be machined by such an apparatus for a given production rate.

Another prior art approach to controlling the radial position of thetool head in a piston turning operation is disclosed in U.S. Pat. No.4,203,062, issued May 13, 1980 to Bathen for "Machine Tool ControlSystems." The Bathen system employs .[.computer.]. numerical controlwith a feedback loop which compares a position signal representing thepresent position of the tool to a programmed position signal to producean error signal which controls energization of a linear motor drivingthe tool.

The present invention is directed to a new and improved turning machinewhich possesses a number of important advantages over prior machines.

One important advantage is that the present invention eliminates themechanical cam and follower type system by using numerical input data todefine the part shape. This data is acted upon by a CNC system whichgenerates appropriate commands to control the cutting tool position atall times during turning. Hence a turning machine embodying principlesof the invention is not limited by the mechanical dynamics of the priorcam and follower systems which established an ultimate limit to themachine's capabilities.

Because the control data is embodied in electronic form in the practiceof the present invention rather than as a mechanical model like a cam,there is no elaborate mechanical change-over required when part shape isto be changed. Rather the CNC is provided with a new part program forthe new part, and it automatically acts upon the new part program datato issue appropriate commands for control of the cutting tool.

Moreover, with the elimination of the mechanical cam and follower, thepresent invention affords the opportunity for attaining even higherdegrees of precision in the high speed turning of parts.

Not only is the versatility of a turning machine significantly enhancedsince it can handle many different part sizes, but with improvedefficiency and precision potentials, the opportunity for significantproductivity gains is also presented by the present invention.

The general idea of applying a CNC system to a cutting tool is of courseknown. For example CNC lathes are representative commercial products.However in the context of a high speed turning apparatus such as apiston turning machine, the application of CNC technology has heretoforebeen deemed impractical because of inherent mechanical limitations inmechanism for positioning the cutting tool.

Consider a situation where a part is to be turned at say severalthousand RPM and is to be provided with an elliptical cross sectionalshape. The cutting tool must make two reciprocations radially of thepart for each complete revolution of the part. In the case of a pistonrotating at 2400 RPM, this means that the cutting tool is required toexecute precisely controlled linear oscillations at a frequency of 80hertz. For example if it is assumed that the acceleration of the tool isrequired to follow a 0.007 inch radial displacement at this speed, theacceleration amounts to 37 feet per second per second. In order toachieve this magnitude of response, the mass associated with theoscillating cutting tool must be small. Yet at the same time that themass, including the cutting tool, is executing this oscillatory motion,they are being subjected to a load imposed by the interaction of thecutting tool with the rotating part. The requirements of minimizing themass associated with the cutting tool in order to attain a satisfactoryresponse at the expected oscillatory frequencies and of accuratelylinearly guiding same with minimum static and dynamic friction, areseemingly inconsistent with requirements that the cutting tool and itsassociated mass be sturdily constructed and supported to react the loadsimposed on them without undesired effects such as tool chatter and/ordeflection so that the desired contour of the part can be achieved.Moreover, since many parts are of complex contour including an axialtaper, such taper usually has to be taken into account as well.

Accordingly, another aspect of the present invention involves a new andunique construction for the mechanical mechanism which oscillates thecutting tool. Among the new and unique features are the prime moverwhich is utilized to impart oscillatory motion to the cutting tool, theconstruction of the cutting tool carriage, and the arrangement forguiding the carriage on a head.

In the preferred embodiment of the invention the prime mover comprises alinear motion, sometimes referred to as a voice coil motor. This primemover has a low inertia armature for fast response, yet it is capable ofprecise movement while exerting ample force to counter cutting loadsimposed when the cutting tool interacts with a part being turned. Thecutting tool carriage is operated by the motor armature. A sturdy, yetlow friction, mounting of the carriage on the head also assists inreacting the cutting loads while enabling the desired oscillatory actionto be obtained so that accurate parts are consistently produced.

Another aspect of the invention involves the cooperative relationshipbetween the CNC system and certain mechanical mechanisms of the machine.A portion of the CNC operation is devoted to a closed loop control withthe part being turned whereby the relative axial position of the part tothe cutting tool and the rotational position of the part about the axisof its rotation are precisely controlled and known at all times. The CNCacts upon the part program in conjunction with the aforementioned closedloop control to issue correlated commands for use in controlling thevoice coil motor, and hence the radial oscillation of the cutting tool.These commands are transmitted by via a high speed data link to aposition profile computer which translates the commands into anappropriate form for causing the voice coil motor to produce the doubleoscillation of the cutting tool per each revolution of the part when anelliptical contour is being machined.

The position profile computer is a system dedicated to the radialpositioning of the cutting tool, and it forms a portion of a closed loopcontrol for the cutting tool position. Associated with the voice coilmotor and carriage, are various sensors which provide feedback signalsto this latter closed loop control. These are all operatively relatedsuch that digital data from the CNC and feedback signals from thevarious sensors are appropriately processed to produce a control currentfor the voice coil motor which produces the desired oscillation of thecutting tool.

Accordingly the dedicated system comprises digital circuit componentsperforming digital calculations. It also has digital-to-analog devicesorganized and arranged to act upon certain digital commands to producean appropriate analog control current for the voice coil motor. As willbe seen .[.leter.]. .Iadd.later.Iaddend., there are particularrelationships involved in this closed loop control which areadvantageous in causing the tool to faithfully follow the CNC digitalcommands.

The foregoing features, advantages, and benefits of the invention, alongwith additional ones, will be seen in the ensuing description and claimswhich should be considered in conjunction with the accompanyingdrawings. The drawings disclose an exemplary, presently preferredembodiment of the invention according to the best mode contemplated atthe present time in carrying out the invention.

BRIEF DESCRIPTION OF THE DRAWINGS

FIGS. 1 and 2 are diagrammatic views, an end view and a longitudinalview respectively, of a complex surface useful for purposes ofexplaining principles of the present invention.

FIG. 3 is a view representing a mathematical definition of the surfacerepresented by FIGS. 1 and 2.

FIG. 4 is a view similar to FIG. 3 corresponding mathematical definitionof tool position with the mathematical definition of part surfaceportrayed by FIG. 3.

FIG. 5 is a diagrammatic view useful in conjunction with FIG. 4 inexplaining a representative complex turning operation.

FIG. 6 is a general schematic block diagram of a machine embodyingprinciples of the present invention.

FIG. 7 is an enlarged view taken generally in the direction of arrows7--7 in FIG. 6.

FIG. 8 is a front elevational view, partly in section, of a portion ofthe machine and can be considered as taken in the direction of arrows8--8 in FIG. 6.

FIG. 9 is an enlarged fragmentary view taken generally in the directionof arrows 9--9 in FIG. 8, but with portions broken away for illustrativepurposes.

FIG. 10 is a transverse cross sectional view taken generally in thedirection of arrows 10--10 in FIG. 9.

FIG. 11 is a view showing a component in greater detail.

FIGS. 12A, 12B and 12C should be considered together and constitute ablock diagram illustrating a portion of FIG. 6 in greater detail.

DESCRIPTION OF THE PREFERRED EMBODIMENT

The reader's comprehension of principles of the present invention whichwill be explained can perhaps be expedited by considering first adescription of certain relationships which are disclosed with referenceto FIGS. 1 through 5.

FIGS. 1 and 2 are diagrammatic views which illustrate a complex surface20 which is typical of a piston skirt. It is to be appreciated thatFIGS. 1 and 2 are diagrammatic in nature and therefore exaggerated inproportion to what would typically be the proportions in an actualpiston.

Surface 20 may be considered to comprise a longitudinal axis 22. Whileit also may be considered as having a generally frusto-conically taperedshape, the actual cross section through the surface is elliptical, ascan be seen from consideration of FIG. 1 which is representative of bothan end view and a typical cross section.

Surface 20 may be mathematicaly defined in any of a number of possibleways. Because the disclosed preferred embodiment of the presentinvention utilizes a CNC, surface 20 is defined as a set of discretepoints in space. These points are most conveniently identified in termsof a three dimensional coordinate system wherein one coordinaterepresents the angular location about axis 22 as referenced from aradial datum another coordinate represents longitudinal (i.e. axial)location along the length of axis 22 as referenced from a longitudinaldatum, and the third represents the length of a radial to axis 22. Forconvenience, the angular coordinate is identified by the general symbolθ, the longitudinal coordinate by the general symbol z, and the radialby the general symbol r.

The degree of precision in the discrete point definition obviouslyinvolves the number of points used. In other words the finer theresolution, the more precise the surface definition.

For convenience let it be assumed that there are a total of n incrementsaround axis 22 so that each angular increment corresponds to 360/ndegrees. In the longitudinal direction, the increments are typicallyquite small, generally thousandths of an inch or less.

On this basis it can be appreciated that at each z coordinate, surface20 is defined by a sub-set of n data points each corresponding to aradial at each of 360/n increments about axis 22. Stated another way,the part surface definition at each z coordinate may be considered as aone dimensional matrix of 360/n radials. If there are m longitudinalincrements, the entire surface is defined by a two-dimensional n×mmatrix of the radials.

In the disclosed embodiment n=360 so that there are 360 one degreeincrements about axis 22. In other words n runs from zero to 359 foreach of m axial increments along the length of axis 22. The number of mincrements depends upon the relation of the turning speed of the partand the axial feed rate of the tool as it makes its pass axially alongthe part and also to the part length. FIG. 3 illustrates thismathematical n×m matrix definition of surface 20.

In the case of a complex piston, the turning machine of the presentinvention operates on a rough piston to cut the skirt to the size andshape of surface 20. The turning machine utilizes a mathematical surfacedefinition such as that represented by the matrix of FIG. 3 to generatethe appropriate motion of the cutting tool relative to the piston. Themeans by which this is accomplished will be explained in the laterdescription. For now, a further diagrammatical description of how thepart surface definition matrix is used in establishing the cutting toolmotion is given with reference to FIGS. 4 and 5.

As the piston turns about axis 22, its angular position about axis 22 iscontinuously monitored. Because the angular position of the cutting toolrelative to axis 22 is known, and is essentially constant if the cuttingtool tip oscillates substantially coincidentally with a radial to axis22, the monitored angular position can be used to determine the angularcoordinate of the part which is being presented to the cutting tool tipat any given instant of time as the piston rotates.

Similarly if the longitudinal position of the part relative to thecutting tool is continuously monitored, the axial location of thecutting tool tip relative to the piston is also known at any giveninstant of time.

Therefore at any given instant of time, these two conditions define thepoint on the piston which is being presented to the cutting tool tip.The turning machine of the present invention operates to act upon thesetwo conditions, and the particular part program being executed by theCNC, to continuously position the cutting tool to the appropriate radiallocation so that the desired surface is cut in the piston skirt.

This relationship is portrayed mathematically with reference to FIG. 4.The tool radial position is called the tool position profile and it isshown to be a function of the part surface definition matrix, and axialand angular positions of the tool relative to the part. The axial andangular positions are, of course, inherently also related to therespective axial and angular velocities.

For convenience let it be assumed that the z-axis velocity, or feedrate,of the tool relative to the workpiece is constant and let it be furtherassumed that the surface which is to be cut in the part is a complex onelike that described with reference to FIGS. 1 and 2. In order to cut anelliptical contour, it will be perceived that for each 180 degrees ofrotation of the part, the cutting tool must make one complete radialreciprocation relative to the axis 22, i.e. one full oscillation. Inother words for each revolution of the part, it is necessary for thetool to reciprocate radially inwardly and outwardly twice because of thenature of the elliptical contour.

For further convenience in description let it be assumed that thegeneral variable x represents the radial tool position relative to axis22. For the illustrated example where there are 360 data points oncearound the circumference of the part, the control operates to generate acorresponding sequence of 360 data points for the tool position. Inother words for the mth subset of the part position matrix, the controlgenerates a corresponding set of data points for the variable x, i.e. itgenerates the tool position profile.

FIG. 5 illustrates the relation of these x data points to theoscillating radial motion of the cutting tool. If it is assumed that thepart is circumferentially located such that one end of the major axis ofthe elliptical cross section is presented to the cutting tool tip, thecutting tool must advance radially inwardly over the ensuing 90° of partrotation since the minor axis of the ellipse is 90 degrees from themajor axis. Because 90 degrees of part rotation provides 90 data points,there is a corresponding set of 90 data points generated for thevariable x and these are depicted in FIG. 5 as occurring along theimaginary straight line segment 24. When an end of the minor axis of theellipse is presented to the cutting tool tip, the cutting tool must thenreverse direction so as to move radially outwardly during the next 90degrees of part rotation. A second set of 90 data points defining thepart surface between 90 and 179 degrees of part rotation causes acorresponding set of 90 data points to be generated for the variable x.These 90 data points are indicated along the imaginary line 26 in FIG.5. Based upon the description just given, the reader will appreciatethat the cutting tool has executed one full oscillation during 180degrees of part rotation. The secon oscillation is portrayed by thesegment 28 with its x data points being generated by the part rotationbetween 180 degrees and 269 degrees and by the segment 30 with its xdata points generated by quadrant of part rotation from 270 degrees to359 degrees.

FIG. 5 is an exaggerated form for illustrative purposes, and it showsthat axial taper is being imparted to the part because each oscillationof the tool moves progressively increasingly radially inwardly. Thistaper is represented by drawing an imaginary line 32 through theradially innermost points of tool tip travel during each oscillation anda corresponding line 34 through the radially outermost points. If therewere no taper to the part the lines 32, 34 would be parallel to axis 22.

The process continues in this manner until the entire part surfacedefinition matrix has been processed by the control to produce acorresponding pattern of motion to the cutting tool resulting in thecreation of the desired shape of the part being turned.

The x data points which are generated by the control are acted upon toproduce the corresponding motion of the cutting tool. Although the datais presented in digital form, the physical characteristics of themachine including both its mechanical and electronic components, coactsuch that a smooth machining action results. This is accomplishedthrough the appropriate selection of the increments and/or through thecharacteristics of certain components of the system; for example in theelectronics digital-to-analog conversion may be used.

With this description of how the tool operation is mathematicallyrelated to the part surface definition, details of the machine itselfcan now be considered.

FIGS. 6 and 7 illustrate in a general way the overall organization andarrangement of a presently preferred embodiment of turning machine 40according to the invention. The machine is illustrated for use inturning the skirt of a piston 42 which is suitably co-axially chuckedand rotated about an axis 44 by means of a drive 46 and a live centerspindle 48. This arrangement for chucking and rotating a part isconventional.

The rotation for drive 46 is delivered by a servomotor 50. A tachometer52 and an encoder 54 are operatively coupled with the servo motor todevelop electrical signals which are utilized by machine 40. Tachometer52 provides a signal representative of the instantaneous velocity ofrotation, i.e. the turning speed, while encoder 54 provides a signalindicative of the instantaneous rotational position. The encoder andtachometer are conventional devices, and although it is known that theposition and speed of a rotating shaft are mathematically related, it isdeemed preferable to utilize two separate sensors to provide therespective speed and position information, rather than a single sensor.

By way of example, encoder 54 may be a digital device which provides adigital signal indicative of the instantaneous rotational position inany convenient unit of measurement. For convenience the digital signalmay be provided in terms of one degree increments of rotation about axis44 and in this way is representative of instantaneous rotationalposition of piston 42 as it rotates about axis 44 relative to acircumferential point of reference. It will be appreciated that thesignal repeats every complete revolution, but that during a revolution,each one degree increment is uniquely identified. By the propercircumferential mounting of piston 42 on the live center spindle andpart drive, the point on the circumference of the piston skirt which isbeing presented to the tip of a cutting tool 56, is always correlatedwith the signal provided by encoder 54 so that the encoder signal, atany instant of time, uniquely identifies the circumferential coordinatewhich is being presented to the cutting tool tip.

The signal from encoder 54 is supplied to a CNC control 58. The mannerin which the encoder signal is acted upon by CNC control 58 will beexplained later.

The signal from tachometer 52 is fed back to a servo amplifier 60 whichcontrols the speed of servo motor 50. The servo amplifier 60 is of aconventional construction to perform a closed loop control of the servomotor with the tachometer 52 providing velocity feedback informationutilized in the closed loop control.

The command input to servo amplifier 60 is delivered by CNC control 58,and is to establish the appropriate turning speed.

As will be appreciated from the early introductory description, tool 56is caused to do an axial pass along the skirt of piston 42, and it isalso concurrently caused to execute small radial oscillations relativeto axis 44 to impart eccentricity to the skirt.

The axial component of motion of the tool relative to the part isprovided by means of a slide 62 which is fed in a direction parallel toaxis 44. For convenience this is identified as the z-axis .Iadd.or thefeed direction.Iaddend..

The z-axis feed is performed by means of a servo motor 64 which isoperatively coupled with slide 62 by any suitable mechanical mechanism,for example a ball screw and nut.

Servo motor 64 is a conventional device, and its shaft position andspeed are monitored by an encoder 66 and a tachometer 68 in a similarmanner to the monitoring of servo motor 50 by tachometer 52 and encoder54. Tachometer 68 and encoder 66 provide feedback information to CNCcontrol 58.

The CNC control forms a part of the closed loop control of servo motor64 by issuing appropriate signals to a servo amplifier 70 which in turncontrols servo motor 64. The CNC control receives a program input whichestablishes the appropriate z-axis feed rate for the particular partinvolved, and the closed loop control operates to control the speed ofservo motor 64 such that the appropriate z axis speed rate for slide 62,and hence like feedrate for cutting tool 56 axially along the pistonskirt, are produced.

The radial component of motion for the cutting tool, which forconvenience will be referred to as x-axis motion .Iadd.or motion in thedepth-of-cut direction.Iaddend., is imparted by a closed loop systemwhich includes electronic and mechanical components. These will bedescribed in considerably greater detail later on. With reference toFIGS. 6 and 7 they are defined generally to include a linear .Iadd.servo.Iaddend.motor 72 which is carried on slide 62. In this way the combinedz and x axis motions are imparted to the cutting tool by means of thecombined operation of servo motor 64 and linear motor 72.

Linear motor 72 is a part of another closed loop control system which isused to achieve precise control of the x-axis motion of the cuttingtool. Instantaneous position is monitored by a linear positiontransducer 74 and instantaneous velocity by a linear velocity transducer76. These two transducers provide feedback signals to a servo amplifierand closed loop control 80 which controls linear motor 72. The inputcommand .Iadd.signal .Iaddend.to servo amplifier and closed loop control80 is received from CNC control 58 via a high speed data link 82. Anoperating panel 78 is associated with CNC control 58 and is adapted toreceive the part program which is to be executed.

Briefly, CNC control 58 issues commands .Iadd.or an initial controlsignal .Iaddend.developed from a program defining the part surface to begenerated, for example as described above with reference to FIGS. 1, 2and 3, and servo amplifier and closed loop control 80 acts upon thecommands to provide .Iadd.or develop a .Iaddend.corresponding.Iadd.final .Iaddend.control .[.signals.]. .Iadd.signal .Iaddend.to thelinear motor 72 to achieve the desired control of the tool x-axisposition. For example, in the case of an elliptical contour, servoamplifier and closed loop control 80 serves to generate two oscillationsof cutting tool 56 for each revolution of the piston 42.

As can be seen in FIG. 7 the cutting tool tip may be slightly offsetfrom the center line of the linear motor, but the x-axis motion iseither exactly or at least substantially along a radial relative to axis44. In this way, the position of the tip of the cutting tool is causedto describe the desired surface which is to be imparted to the pistonskirt.

FIGS. 8, 9, 10, and 11 illustrate in detail the mechanism by whichx-axis motion is imparted to cutting tool 56. The mechanism comprises ahead 82 which is mounted by any suitable means on slide 62. Head 82comprises a base plate 84, a rear plate 86, a top cap 88, and a topcover 90 assembled together and forming an enclosure for linear motor 72and the two transducers 74, 76.

As can be seen from consideration of FIGS. 8 and 10, base plate 84comprises a horizontal bottom wall 92 and upright side walls 94, 96.Hence, as viewed lengthwise of the x-axis in FIG. 10, base plate 84 canbe considered to have a generally U-shaped cross section.

At the forward end, i.e. the right hand end as viewed in FIG. 8, baseplate 84 is covered by top cap 88; the rear, or left hand, end isenclosed by top cover 90. Linear motor 72 is enclosed within the rearportion of the head and has an axis 97 along which it acts.

Linear motor 72 comprises a magnet assembly 98 and a coil assembly 100.Magnet assembly 98 has an annular shape and an axis concentric with theaxis 97. Coil assembly 100 also has an annular shape and is coaxial withmagnet assembly 98.

Magnet assembly 98 comprises a frame 102 of generally annular shapehaving a pair of bars 104, 106 (see FIG. 10) fixed to the outside toprovide for its mounting on base plate 84. These bars rest on ledges108, 110 of base plate 84, and frame 102 is securely attached to thebase plate by screws 112 passing through suitable holes in the baseplate bottom wall and into tapped holes in the bars 104, 106.

Magnet assembly 98 further comprises a magnet 114 disposedconcentrically within frame 102. Magnet 114 has a circular annular shapeand is of a length less than that of the frame. It is affixed within theframe in any suitable manner so as to be coaxial with axis 97.

Magnet assembly 98 is enclosed at its rear axial end by means of an endplate 116 attached to frame 102. A central cylindrical hub 118 ismounted on and projects forwardly from end plate 116. In cooperationwith magnet 114, hub 118 defines a circular annular free space 120, andit is within this annular free space that a rear portion of coilassembly 100 is disposed.

Coil assembly 100 comprises a circular tubular wall 124, or bobbin,which attached by means of a cap 126 at its forward end to a carriage128. Carriage 128 in turn projects forwardly from cap 126 to terminateat a forward end containing a suitable tool mount 130 for tool 56.

Carriage 128 is illustrated as a hollow tubular bar having an axiscoincident with axis 97 and having a square transverse cross sectionalshape as best seen in FIG. 10. The carriage axis is parallel to thex-axis, and the carriage is arranged for axial motion by linear motor72. Carriage 128 is accurately guided on head 82 by sets of rollers. Therollers provide for low friction mounting, yet are sufficient to reactcutting loads so that at all times during turning operations the cuttingtool is enabled to accurately follow commands and cut a desired surfaceon the part being turned.

The rollers for guiding carriage 128 are arranged in sets. Forconvenience these are referred to as a vertical acting set and ahorizontal acting set. The carriage is vertically constrained by thevertical set of rollers, and it is horizontally constrained by thehorizontal set of rollers.

The vertical set is seen with reference to FIGS. 8 and 10 where it isshown to comprise a lower half-set 132 and an upper half-set 134. Theupper half-set 134 is spring-loaded while the lower half-set 132 is not.

The upper half-set comprises four individual circular rollers, orwheels, 138 of identical size. Two of these rollers are on the ends of arear axle 140 while the other two are on the ends of a forward axle 142.The rear axle 140 is supported on a rear yoke 144 and the forward axle142 on a forward yoke 146. The two yokes 144, 146 are joined together bya leaf spring assembly 148 which extends axially between a forwardportion of rear yoke 144 and a rearward portion of forward yoke 146. Theattachment of the leaf spring assembly to the yokes is by any suitablemeans, such as by screws passing through apertures in the leaf springassembly and into tapped holes in the yokes. The leaf spring assembly iscentrally disposed, as viewed in FIG. 10.

The leaf spring assembly is further provided with one or more apertureslocated centrally of its length for attachment to the underside of topcap 88 as at 150. The attachment is made by any suitable means such asone or more screws passing through one or more apertures in the leafspring assembly and into a corresponding tapped hole or holes in top cap88. Spacers may or may not be used, as required.

The lower half-set 132 comprises a total of six rollers 138. Two ofthese rollers are in the rear and mounted on an axle 156 which is belowaxle 140. The remaining four rollers of the lower half-set are arrangedin pairs on respective axles 158, 160.

The rear axle 156 is mounted on a yoke which is securely fastened to aledge 162 of base plate 84. The two forward axles 158, 160 are on a yoke164 which is capable of a certain amount of adjustment.

A pair of holes are provided in the bottom surface of yoke 164 and thedistal ends of a corresponding pair of screws 166 pass into these holes.Details of each screw 166 are shown in FIG. 11. The distal end of thescrew is rounded at 168 to provide a bearing surface on which thefrusto-conically tapered end of the corresponding yoke hole seats. Thescrews are threaded into tapped holes in base plate 84 so that eachscrew is capable of being vertically adjusted. In this way it ispossible to vertically position the forward set of four rollers. Once adesired adjustment has been obtained, the screws are locked by means ofjam nuts 170. The screws and jam nuts are accessible via suitable tools(not shown) which are introduced into a recess 172 to obtain the access.After adjustment and locking of the screws, recess 172 may be closed bya suitable plug 172a.

Thus, the upper half-set 134 comprises four rollers which exert downwardforces on the carriage at spaced apart points, and the lower half-set132 provides subjacent support. The top cap is shaped on its interiorface to accommodate the upper half-set of rollers. The magnitude of thespring-loading is a function of the leaf spring characteristics and theamount of deflection thereof. Specific details of any given constructionwill depend upon the cutting loads anticipated, with the spring forcebeing sufficient to prevent load-induced deflections. It is contemplatedthat a leaf spring assembly may comprise single or multiple leaves.

This arrangement provides a secure mounting of the carriage on the head,for guidance and load reaction purposes, yet allows the carriage toreciprocate axially along the axis with minimal resistance.

The horiziontal set of rollers is analogous to the vertical set in thatit comprises two half-sets, one to each horizontal side of carriage 128.As viewed in FIG. 10, one half-set 174 is to the right side, and theother half-set 176 is to the left side.

The half-set 174 is analogous to the lower half-set 132 just describedand the half-set 176 is analogous to the upper half-set 134. Each of theten rollers in the horizontal set is identified by the reference numeral178. They are arranged such that there are six rollers 178 in half-set174 and four rollers 178 in the half-set 176. The six rollers ofhalf-set 174 are arranged in exactly the same manner as the six rollers138 of the lower half-set 132; in other words, one pair are on a yoke atthe rear and the other two pairs on a forward yoke. The forward pairsare laterally positionable relative to the carriage axis in the samemanner as yoke 164 is vertically positionable. The two screws and jamnuts for laterally positioning the two forward pairs of rollers ofhalf-set 174 are designated by the reference numerals 182 and 184respectively. After the appropriate adjustments and locking have beenmade, access is prohibited by a cover plate 186 attached to the outsideof base plate 84. (FIG. 9).

The half-set 176 is spring-loaded in the same manner as the upperhalf-set 134, comprising a leaf spring assembly 188 attached centrallyof its length to the side wall 94 of base plate 84. With thisarrangement, the horizontal set confines carriage 128 horizontally forguidance and load reaction purposes, yet allows the carriage toreciprocate axially with minimal resistance.

A protective bellows 190 serves to seal the interior of the head aroundthe carriage in the area where cutting activity takes place. The bellowscomprises a smaller four-sided aperture at its forward end which fits ina sealed manner around the forward end of carriage 128 and extendsrearwardly to a layer fourside rear aperture which attaches via amounting ring 192 to the front of head 84 in a sealed manner. Thebellows is constructed from a sturdy and durable material for protectivepurposes, yet it has sufficient flexibility that it imposes nosignificant restriction in the translation of the carriage on the head.

Further details of linear motor 72 will now be described. A layer ofcopper 204 is applied onto the outside of hub 118 and a soft copper ring206 located as shown at the joint between frame 102 and end plate 116.Magnet 114 is polarized to issue a magnetic flux which passes throughfree space 120. A very uniform magnetic field is created in the freespace 120, and it is within this free space that an electric coil 200 ofcoil assembly 100 is disposed.

Coil assembly 100 may comprise a suitable slot 202 within which the coilis securely disposed. The coil is energized with an electric current anddepending upon the magnitude of this electric current, there will be acertain degree of interaction between the magnetic field created bymagnet 114 in free space 120 and the magnetic field created by theelectric current flowing in the coil. The consequence is that an axialforce is applied to the coil, and hence also exerted on the entire coilassembly and carriage. This force is effective to selectively positionthe carriage with the positioning being a function of the magnitude ofthe current introduced into coil 200. In other words, by controlling thecurrent in coil 200, the motion of carriage 128 is also controlled, andin the present invention the reciprocation of the cutting tool istherefore controlled by control of the current in coil 200. Wires fromcoil 200 extend to a plug 203 via which a connection is made to thecontrol.

The two sensors 74 and 76 associated with linear motor 72 are alsocontained within the interior of head 82. The LVT sensor 76 is embodiedas a coil 208 disposed on a tube 210 which is inserted coaxially througha suitable hole in end plate 116 and hub 118. The hole in the end plateand hub may be larger than the OD of the tube and coil and thereforebushings 212 may be used at the ends to securely support the tube. Acore 214 is disposed within tube 210 and is connected by a rod 216 tocap 126. Reciprocation of carriage 128 by linear motor 72 causes asimilar reciprocation of core 214 within tube 210. This develops asignal in coil 208 corresponding to the instantaneous velocity and thecoil is connected via lead wires to a plug 218 via which a connection ismade to the control.

The LPT sensor 74 is located more forwardly. This sensor is a highprecision device capable of very fine resolution. An example of such adevice is Heidenhaim sensor. It comprises a scale 220, in the form of agrating, attached to carriage 128, and a sensing head 222, fixedlymounted within head 82 in confrontation with scale 220. Lead wiresextend from sensing head 222 to a plug 224 which delivers the LPT signalto control 80.

There are two additional sensors also located within the head. Onesensor is a home sensor for establishing a home position for thecarriage, and the other is an on-scale sensor for sensing when the scale220 is active on the sensing head 222. (These are portrayedschematically in FIG. 12C, to be described later).

The range of travel of carriage 128 on the head is greater than thelimited range within which complex turning operations are conducted.Hence the precision LPT sensor 74 is active only over a limited extentof the total possible range of travel of the carriage.

At the beginning of a turning operation, the carriage is extendedforwardly from the home position to a position where actual cuttingtakes place. In this regard it can be appreciated that the home sensorand the on scale sensor provide the appropriate control functionswhereby the carriage may rapid advance from the home position until itcomes on scale at which time sensor 74 assumes control and precisionturning operations are conducted. At the conclusion of the precisionturning operations, the carriage can rapidly retract to the homeposition.

The range of travel of the carriage is limited by crash stops 230 and232. These crash stops are arranged within head 82 to act on cap 126. Aspacer member 234 is attached between frame 102 and the rear end wall oftop cap 88. Crash stop 230 is attached within head 82 where the interiorof spacer member 234 abuts the end surface of top cap 88.

The other crash stop 232 is in the form of a circular annular member 236mounted on the forward end of hub 118 concentrically with the axis. Therod 216 which connects from cap 126 to core 214 passes centrally throughcrash stop 232. The rearward travel of the carriage is limited byabutment of the rear axial face of cap 126 with crash stop 232, andforward travel by abutment of the chamfer at the forward perimeter ofcap 126 with crash stop 230. The purpose of the crash stops is toprevent undesirable overtravel of the carriage, and in the usualmachining operations they do not come into play. Rather they come intoplay such as when improper control is introduced into coil 200 whichotherwise would cause undesired, and potentially damaging, overtravel.

All three plugs, 203, 218, and 224 are mounted in top cover 90, and eachmates with a corresponding connector via which circuit connection ismade to the control.

The top cover is a formed part attached by screws so as to beconveniently removable for access to the interior of the head at therear. The remainder of the head structure is constructed of sturdy partsto support the carriage and voice coil motor.

In use, head 82 is enclosed. It may be desirable to provide a certainair circulation within the interior but without introducing anyundesired contamination. This can be done by means of an inlet andoutlet port (not shown) for example through baseplate 92, with filteredair being the medium circulated. The various component parts areassembled together in a conventional manner to construct the head. Afterassembly, adjustment of the rollers will typically be required to insurethat the carriage motion follows the intended path. For alignment of thecarriage on the head a pair of holes 242, 244 are provided, and thesemay be used to attach indicators or other pieces of equipment which areused to perform the alignment. Once the desired alignment has beenobtained by the adjustment of the rollers, the adjustment screws arelocked in place by the jam nuts.

Because of concern about the mass of the carriage and those parts whichmove with the carriage, they are made of strong light-weight materials,such as titanium. In order to obtain the best possible characteristicsfor the magnetic field, magnet 114 is constructed from an exotic alloysuch as samarium cobalt.

During turing operations, the current in coil 200 is controlled in sucha way as to create the desired oscillatory motion of the carriage andcutting tool. Hence the control current in the coil will contain anoscillatory component corresponding to the oscillatory motion desired.Exactly how this oscillatory current is developed can be betterunderstood for consideration of FIGS. 12A, 12B and 12C and the ensuringdescription.

FIGS. 12A, 12B, and 12C should be considered together and constitute amore detailed schematic diagram. Looking first to FIG. 12C, the readerwill observe that linear motor 72 and sensors 74 and 76 areschematically portrayed. Also schematically portrayed are the homesensor and on-scale sensor previously referred to and now identified bythe respective reference numerals 302 and 304. These two sensors haverespective amplifiers 306 and 308, and an amplifier 310 is alsoassociated with LVT sensor 76. These components just described are allcontained within head 82. Their operative coupling with control 80 isvia plugs 203, 218 and 224.

Wires 312 and 314 serve to connect linear motor 72 with control 80.Wires 316, 318, 320, 322, 324, and 326 connect the various sensors withthe control. Control 80 contains a relay rack portion 328 and anelectronic portion 329. Some of the wires from head 84 connect todevices in relay rack 328 while others pass through to the electronicportion 329.

Relay rack 328 contains a control unit 330 and a power amplifier 332both of which are associated with linear motor 72. Control unit 330comprises a relay coil 334 controlling a moveable contact 336. Wire 312connects to moveable contact 336 and the contact is controlled by coil334 in the following manner.

In one condition of coil 334, the contact 336 is connected (as shown) toa wire 338 leading from power amplifier 332. In other words in thiscondition of coil 334, power amplifier 332 controls linear motor 72.

When coil 334 is operated to another condition, contact 336 is moved toconnect wire 312 to another wire 340 leading to a voltage reference V.In this condition, the voltage V controls linear motor 72, causing toolretraction.

Wires 342 and 344 extend from control unit 330 and power amplifier 332respectively. Wire 342 connects to a wire 346 from a motor overcurrentprocessor command 348. The motor overcurrent processor command 348controls the condition of coil 334 and hence controls whether the motoris being operated by power amplifier 332. or by the voltage V.

Wire 344 connects to a wire 350 via which a command signal is suppliedto power amplifier 332. When power amplifier 332 is connected to motor72 by contact 336 being in the condition shown in FIG. 12C, the commandsignal supplied by wire 350 controls the motor. The connection of thepower amplifier and control unit to the wires 346 and 350 is via aconnector and plug 352 which is shown to provide additional connectionsfor other wires between relay rack portion 328 and the electronicsportion 329.

Relay rack portion 328 also contains a multiplier 354 which iscooperatively associated with sensor 74. The three wires 318, 320, and322 are inputs to multiplier 354 and there are three output wires 356,358, 360 from multiplier 354. These latter wires connect throughconnector and plug 352 to respective wires 362, 364, 366.

Sensor 74 .Iadd.or scale reference system .Iaddend.provides outputsignals on the lines 318, 320 and 322 as the scale .Iadd.of the system.Iaddend.moves past the sensing head .Iadd.of the system.Iaddend.. Lines318 and 322 deliver respective squarewaves which are phased 90 degreesapart from each other. As can be appreciated, the frequency of the twosignals is related to the velocity of movement and because of themathematical relationship of distance to velocity position informationis also provided. By including the relative 90 degree phasing, the twosignals delivered by lines 318 and 322 also contain directionalinformation.

The signal delivered via channel 320 is a reference position markersignal representing a predetermined location along the scale. Thislocation is used as an absolute reference position.

Multiplier 354 acts upon the signals to enhance the resolution.Multiplier 354 is a standard device which is manufactured also by thesame company that manufactures sensor 74.

Wires 368 and 370 are connected via connector and plug 352 with therespective wires 324 and 326.

Thus FIG. 12C generally relates the earlier description of theassociated components with the generalized block diagram of FIG. 7.Attention can now be focused on further details of control 80 withreference to FIGS. 12A and 12B.

In FIG. 12A a multibus 380 interfaces control 80 with the CNC 58 viahigh speed data link 82. Multibus 380 serves a number of devices incontrol 80 which are illustrated generally to comprise data receivers382, address decoding 384 and control 386. The inputs to multibus 380are in digital form and collectively define data information, addressinformation, and control information. For example, the data informationrepresents x-axis position information to command positioning of thecarriage; address information identifies particular devices withincontrol 80 which are to receive the data information or controlinformation; and control information in conjunction with addressinformation controls the flow of data information within control 80, orcontrol information can issue direct commands to certain devices. Ablock labeled command latch 388 can latch data. It will be appreciatedthat these devices have been generally portrayed in FIG. 12A and that inthe actual implementation of a control, there are specific linesconnecting the various devices so that the inputs received on multibus380 are properly utilized.

Still referring to FIG. 12A, it will now be explained how x-axis controlof the carriage is accomplished. Input data representing tool x-axisposition commands is input to a position buffer latch 400. Positionbuffer latch 400 in turn connects to a current position latch 402 whichitself in turn connects to a prior position latch 404. In operation, theflow of input data is sequentially from latch 400 to latch 402 to latch404. In other words data is sequentially moved from a preceding latch toa succeeding latch. The rate is a function of the speed at which thepiston is being turned and the value of n. If it is assumed that n=360and the piston turning speed is 40 r.p.s., then the rate of data flow is14400 hz. Thus the latches 400, 402, 404 may be considered to form achannel with the output of latch 404 providing an instantaneous demandposition in digital form.

A digital-to-analog converter (DAC) 406 converts the digital demandposition into an analog one. This is supplied as an input to a summingjunction 408. For convenience the instantaneous demand position will bedesignated x_(i).

A gate 410 receives two inputs entitled "Processor" and "Interrupt". Theoutput of the gate in turn connects to latches 402, 404. Under normalturning operations, data passes sequentially through the channel in themanner just described. However certain conditions may call forinterruption and this is done by the actuation of gate 410 acting uponthe latches 402, 404.

The processor and interrupt signals represent control signals receivedfrom the CNC 58 and/or control panel 78.

The output of latch 402 is supplied to one input of a digital subtractcircuit 412 while the output of latch 404 is supplied to the other inputof the digital subtract circuit. The digital subtract circuit subtractsthe two signals to yield an output signal corresponding to thedifference between the current position and the prior position asregistered by the two latches 402, 404. At any instant of time theoutput of digital subtract circuit 412 represents the size of the nextincrement of x-axis motion to be commanded.

The output of circuit 412 is supplied as an input to a DAC 416. Areference input to DAC 416 is supplied from a second DAC 414, and thissecond DAC receives certain data. The output signal from DAC 416 issupplied both to an integrator 418 and to a circuit 420. The integratoroutput is connected to a second input of summing junction 408. Theoutput of circuit 420 is connected to an input of a further summingjunction 422 (see FIG. 12B). The output of summing junction 408 is alsoan input to summing junction 422.

The circuit containing integrator 418 is selectively used depending uponcertain conditions. Basically, when used, it is intended to perform asmoothing function whereby the output of summing junction 408 may beconsidered to constitute a demand position "smoothed" which will resultin smoother motion of the carriage.

Circuit 420 performs the transfer function sK₂ on x_(i), s being thewell-recognized symbol for the Laplace operator used in mathematicaldescription of servomechanisms.

Circuit 420 provides a feed-forward signal to enhance the response ofthe carriage to the basic demand position. The basic demand position istransmitted from latch 404 to DAC 406 which in turn connects throughsumming junction 408 onto summing junction 422. Position feedback issubtracted at summing junction 422, and how the position feedback isdeveloped will be explained later.

DAC 406 has a gain K_(i) acting on x_(i). Therefore, assuming thatintegrator 418 is inactive, the input signal to summing junction 422 is(K_(i) +sK₂)x_(i). It is from this signal that the position feedbacksignal is subtracted at summing junction 422 to produce a position errorsignal used to control linear motor 72. How the position feedback signalis developed will now be explained.

Multiplier circuit 354 connects to receivers 424 via lines 362, 364,366. An output line 426 from receivers 424 is an input to a fiduciaryup/down counter 428. Two other lines 430 and 432 from receivers 424 areinputs to a direction logic circuit 434. Line 432 also connects to aninput of counter 428. An output line 436 of direction logic circuit 434is returned to another input of counter 428.

The fiduciary up/down counter 428 is intended to set an absolutereference point which is called zero reference. The zero reference isset by response to the marker signal developed on line 358 andtransmitted to line 364, as explained earlier. When the marker occurs,fiduciary up/down counter circuit 428 is enabled to begin counting fromzero with the count being a measurement of the travel of the carriagefrom the reference zero. The direction logic 434 provides the properdirection of counting so that the counter faithfully follows thecarriage travel in both directions along the x-axis. Because the markerconstitutes an absolute reference on the machine, the control is nowrelated to the absolute reference.

A cutting start position register 440 is preloaded with dataconstituting an absolute position at which turning operations are tocommence. It will be appreciated that this preload will typically be setto take into account the expected size of the part before turning sothat the commencement of turning operations is slightly off the part toavoid the cutting tool inadvertently plunging into the part.

The cutting start position data in register 440 is compared with theinstantaneous position registered in fiduciary up/down counter 428 by acompare circuit 422. The compare circuit has an output line 444 whichcauses a cutting up/down counter 446 to begin to follow the carriagetravel once the carriage has traversed the offset preloaded in register440. Counter 446 thereby takes into account the offset which has beenpreloaded in the cutting start position register 440 relative to theabsolute reference of counter 428.

The outputs from the two counters 428 and 446 are supplied respectivelyto latch circuits 452 and 454 respectively. The latch circuits provideinformation on the multibus which is made available to the CNC.

Since the output of counter 446 represents the travel of the carriagefrom the cutting start position, it can be used to provide positionfeedback information for the closed loop control. This position feedbackis designated in the FIG. 12B as x₀ and is supplied to a DAC 456 whichhas a gain K_(p). The output from DAC 456 is an analog measurement ofthe instantaneous carriage position as measured from the offset. Thisinformation is processed by a circuit 458 which imposes the transferfunction

    1/1+sT

on it, and in turn connects to one input of a summing junction 460.

Velocity feedback from sensor 76 and its amplifier 310 is transmittedthrough an amplifier 462 to a circuit 464 which imposes the transferfunction

    1/1+sT

on it. The resulting signal is also supplied to a summing junction 460.The sum of the two input signals to summing junction 460 is subtractedat summing junction 422 from K_(i) x_(i) to close the position feedbackloop. Hence although the position feedback is composed principally ofposition information, it contains a component of velocity informationalso.

The error signal from summing junction 422 is supplied through a contact450 to a further summing junction 466. Contact 450 is controlled by adevice 448, and in this regard both device 448 and 450 could besolid-state, as well as the electromechanical depiction of the drawing.Device 448 is activated by the enablement of counter 446, and whenactivated it causes contact 450 to provide continuity from summingjunction 422 to summing junction 466. This represents the commencementof closed-loop position control at the beginning of turning operations.

The velocity feedback from amplifier 462 is also supplied to thesubtraction input of summing junction 466 and the output of the summingjunction is an error signal supplied to an amplifier 468. It is thisamplifier 468 which in turn supplies the command to power amplifier 332.

It is at summing junction 422 that the position error signal is createdby subtracting the signal from summing junction 460 from the signal fromsumming junction 408. Assuming that the integrator 418 is inactive, thedemand position to summing junction 422 is the sum of the signal K_(i)x_(i) from DAC 406 and the signal sx_(i) K₂ from circuit 420, as notedabove.

The feedback signal subtracted from the demand signal at summingjunction 422 equals: ##EQU1## where

K_(p) represents the position feedback gain,

K_(v) represents the velocity feedback gain, and

x₀ =v/s, where v represents the instantaneous velocity.

The position error signal is caused to satisfy the following condition:##EQU2## A particularly advantageous relationship is obtained bychoosing the parameter T such that the following relationship issatisfied:

    T=K.sub.v /K.sub.p,

and by also setting

    K.sub.2 /K.sub.i =K.sub.v2 /K.sub.p0.

The selection of the individual circuit components to satisfy theserelationships is accomplished through the practice of conventoinaldesign .[.techaniques.]. .Iadd.techniques .Iaddend.used in electronicand servomechanism design.

When switch 450 is in the position which couples the output of summingjunction 422 to the input of summing junction 466, the control assumesthe position control mode of operation and it is this mode which is usedduring precision turning operations on a part.

From consideration of the foregoing description and the drawings, itwill be further appreciated that in the position control mode ofoperation, the minor feedback loop providing velocity feedback is alsoactive to modify the position error signal. This modification occurs atsumming junction 466 and creates an error signal input to amplifier 468which in turn acts upon power supply 332 to effect corresponding controlof linear motor 72.

During an operating sequence on a part, the position control mode ofoperation may be active for only a portion of the time. A typicaloperating sequence involves the carriage advancing from a home, orretracted, position toward a part with the actual turning operationsbeing allowed to commence only after the cutting tool has been broughtinto close proximity with the part.

Over the range of carriage advance from the home position to a positionjust off the part, the control operates in the velocity control modewhere only the velocity feedback loop is active.

In the velocity control mode, switch 450 is operated to conduct a demandvelocity signal received from the CNC and converted into an analogsignal by a DAC 470 to summing junction 466. The demand velocity is setby the particular part program. The part program also sets the cuttingstart position register with data representing offset, or the point atwhich the position control mode of operation is to commence.

As the tool and carriage advance toward the part while the control is inthe velocity control mode of operation, the reference marker is issuedby position sensor 74 to set the absolute reference point into thecontrol. The velocity control mode will continue until the offset, ifany, has been tranversed.

Once the offset has been traversed, the position control mode begins atwhich time the up/down cutting counter 446 becomes active, and thecontacts 450 are switched to conduct the signal from summing junction422, instead of the signal from DAC 470, to summing junction 466.

The position data received from the CNC control and acted upon bycontrol 80 causes input position commands to be issued to effect theclosed loop position control of linear motor 72 and hence of thecarriage and tool. In this regard the position commands are correlatedwith the rotation of the piston as provided by position encoder 54, andtherefore the tool is caused to follow a path such as described abovefor example with reference to FIG. 4 where in turning an ellipticalcontour the carriage is caused to execute two oscillations per eachrevolution of the piston. The control operates to ensure the faithfulcorrespondence of the cutting tool to the demand position so that thedesired contour is imparted to the piston skirt. The process continuesuntil the program has been executed at which time the carriage can beretracted to the home position.

The turning operations are conducted with efficiency and accuracy. Theinteraction between the control electronics and the mechanical mechanismachieves a response which enables the tool tip to closely track thedesired contour to be created in the part while the part is rotating atrelatively high speed. Moreover, this is accomplished without undesireddeflections, tool chatter or like impediments to the machine'sperformance, and the mechanical construction of the head in conjunctionwith the linear motor are especially advantageous in enabling thisoutstanding performance to be attained.

While the advantageous aspects of the invention are most readilyapparent when the tool is performing the finish operations because thisis where the final accuracy is imparted to the piston, the invention canbe used to conduct operations other than finish turning. For example, byappropriate programming of the CNC it is possible for the tool toconduct semifinish turning, grooving, and other related operations, inaddition to finish turning. Therefore, a turning machine embodyingprinciples of the invention is adapted to conduct all the necessaryoperations which are required in turning of a part such as a piston.Because these operations may be defined mathematically by the computerprogram entered into the CNC, significant accuracies and improvements inefficiencies result while at the same time the machine is endowed withversatility to produce parts of different geometrical requirementswithout any significant changeover.

The CNC is a conventional apparatus which is programmed in aconventional manner to cause the control to execute the above definedfunctions. In this regard conventional programming techniques areemployed to create an operating program based on knowledge of the partgeometry and the organization and arrangement of the turning apparatus.The CNC can perform the necessary calculations on a realtime basis toprovide signals via high speed data link 82 to control 80. For exampleas noted above, 14,400 hertz may be a typical frequency of position datatransmission, and for the typical geometries involved, the mechanism canfaithfully follow inputs at this rate.

A turning machine embodying principles of the invention exhibits aperformance which enables the cutting tool to follow, with quickness andaccuracy, changes which are correlated with the rapidly rotating partbeing turned. Depending upon the actual profile of a part, any givenupdating from the CNC may or may not contain a change in positioninformation. For example, in the case where a circular shape is to becreated in conjunction with an axial taper, the radial position of thecutting tool would change at most only once per revolution of thepiston, and therefore the updating of the demand position would actuallychange at most only once per revolution of the part.

The versatility of the invention should be readily apparent. I order tochange the machine to turn a different piston shape, all that isnecessary is to load the CNC with a new program relating to the newpiston geometry. The CNC will act upon the program in conjunction withthe feedback signal from encoder 54 to provide appropriate commandsignals for the radial positioning of the cutting tool. The closed loopcontrol as disclosed with reference to FIGS. 12A, 12B, and 12C comprisesmeans for enabling the tool to faithfully follow the commands.

It will also be observed that the mechanical construction of the turningmachine has the advantage of being relatively compact, yet possessingstrength to conduct the turning operations in conjunction with theability to respond quickly to even small changes.

Based upon the foregoing description, it will be appreciated thatdetails of the turning machine construction can be specified inaccordance with conventional engineering design and fabricationprocedures. For example the characteristics of the leaf spring mechanismare chosen to provide for the forceful constraint of the bar in theassembled condition of the machine as illustrated with reference toFIGS. 8, 9, and 10. The amount of force which is exerted is sufficientto prevent undesired deflections at all times, even tool chatter whenthe tool is operating on a workpiece in the usual manner. The extent ofthe yieldable forceful constraint is however insufficient to detract inany significant way from the ability of the carriage to roll fully onthe sets of rollers. Although the engagement of the rollers with the baris described as being yieldably forceful, the typical usage of theturning machine under the intended operating conditions does not resultin yielding of the rollers.

It is also to be observed that the action of the rollers on the carriageis symmetrical so that no undesired bending loads are created in thecarriage by virtue of the yieldably forceful action of the rollers uponit. By utilizing a bar of rectangular cross section for the carriage andtwo sets of orthogonally related (90° apart) rollers acting upon theopposite parallel surfaces of the bar, the bar is accurately guided forstraight line motion. With the adjustment feature provided for each ofthe two orthogonally related sets of rollers as described above, theline of travel can be accurately set in both a vertical plane as well asa horizontal one.

It should also be observed that the rollers alone serve to guide andconstrain the moving parts. In other words there is no guide means whichis directly active on the armature of the linear motor.

The invention comprises a significant development in turning apparatus,and while a preferred embodiment has been disclosed, it will beappreciated that principles are applicable to other embodiments.

What is claimed is:
 1. In a turning machine (40) comprising means forrotating (48) a part (42) about an axis (44) and for causing a cuttingtool (56) to make an axial pass along the part while the radial positionof the tool relative to the part is selectively controlled incorrelation with the angular position of the part about the axis ofrotation as the part is being rotated, the improvement which comprises amain CNC control (58), angular position sensing means (54) operativelycoupled with said means for rotating the part to provide to the CNCcontrol information representative of instantaneous angular position ofthe part about the axis for rotation as the part is being rotated,radial position sensing means (74) for sensing the radial position ofthe cutting tool relative to the part, radial velocity sensing means(76) for sensing the radial velocity of the tool relative to the part,and a closed loop control system (80) for closed loop controlling theradial position of the cutting tool relative to the part as a functionof the radial position of the cutting tool and the radial velocity ofthe cutting tool, said closed loop control system comprising an electriclinear motor (72) as the prime mover for radially positioning thecutting tool, said CNC comprising means for issuing commands to theclosed loop control system correlated with the information with saidposition sensing means and information about the part, and said closedloop control system comprising means for converting the CNC commandsinto a corresponding control current for the linear motor .Iadd.whereinone end of an elongated carriage is adapted to be coupled to the cuttingtool, the other end of the carriage being coupled to the motor fortransmitting linear motion and wherein said linear motor and carriageare mounted on the head of the machine, and in which said carriage isguided for reciprocable linear motion on the machine head by guide means(138,178) disposed in cooperative relation with the carriage and themachine head for facilitating sliding movement of the carriage, andspring means (148) for spring-loading the guide means with a springforce sufficient to resist displacement in any direction transverse tothe radial direction and thereby minimize deflection of the carriage inany such direction.Iaddend..
 2. The improvement set forth in claim 1 inwhich said linear motor comprises a linearly moveable armaturecontaining a coil disposed within a magnetic field and said controlcurrent being .[.oonducted.]. .Iadd.conducted .Iaddend.through said coilto control the linear positioning of said armature.
 3. The improvementset forth in claim 2 in which the magnetic field within which said coilis disposed comprises means, including a circular annular magnet,defining an annular free space within which flux issued by the magnet iscontained, and said coil is concentrically disposed within said freespace.
 4. The improvement set forth in claim .[.3.]. .Iadd.2 .Iaddend.inwhich said armature is in the form of a bobbin which in turn isoperatively connected to .[.a.]. .Iadd.the .Iaddend.carriage .[.on whichthe cutting tool is mounted.].. .[.5. The improvement set forth in claim4 in which said linear motor and carriage are mounted on a head of themachine, and in which said carriage is guided for the linear motion onthe machine head by means of guide means, said bobbin being free ofcontact with said guide means..].
 6. The improvement set forth in claim.[.5.]. .Iadd.1 .Iaddend.in which said guide means comprises pluralrollers acting upon said carriage.
 7. The improvement set forth in claim6 in which said carriage comprises a hollow bar having an exteriorsurface which in transverse cross section is polygonal, and in whichthere are plural sets of rollers having rolling contact with theexterior surface of said hollow bar.
 8. The improvement set forth inclaim 7 in which at least one set of rollers comprises pluralnon-yieldably mounted rollers and plural yieldably mounted rollers, suchyieldably and non-yieldably mounted rollers coacting on respectiveexterior surface portions of said hollow bar to provide for yieldablyforceful constraint of said hollow bar by such rollers while stillallowing the carriage to be rolled on said rollers.
 9. The improvementset forth in claim 8 in which said bar's exterior cross sectional shapeis rectangular and there are two sets of rollers, orthogonally related,each set co-acting on respective opposite exterior surface portions ofsaid bar, and in which the non-yieldably mounted rollers of each setcomprise a forward pair and a rearward pair having rolling contact withsaid bar at spaced apart points and the yieldably mounted rollers ofeach set comprise a forward pair and a rearward pair having rollingcontact with said bar at substantially the same forward and rearwardlocations as for the coacting non-yieldably mounted ones, the yieldablymounted roller pairs each being mounted on the machine head by means ofa corresponding leaf spring mechanism, and further including means forselectively positioning at least one pair of each set's non-yieldablymounted rollers toward and away from the axis of the hollow bar. .[.10.The improvement set forth in claim 1 in which the linear motor ismounted on a head of the turning machine and is operatively connected toa carriage on which the cutting tool is mounted, and said closed loopcontrol system including plural sensors on the machine's head operatedby movement imparted to said carriage by said linear motor, said sensorsproviding respective signals for use by the closed loop controlsystem..]. .[.11. The improvement set forth in claim 10 in which one ofsaid plural sensors provides a velocity signal representative ofinstantaneous velocity of said carriage and another of said pluralsensors provides a position signal of instantaneous position of saidcarriage..]. .[.12. The improvement set forth in claim 11 in which saidone sensor comprises a sensing coil concentric with said linear motorand disposed within a bore passing through the interior of the linearmotor, said one sensor further including a core concentrically disposedwith respect to said sensing coil and moveable through the sensing coilwith the operation of the carriage by the linear motor so that thesensing coil provides a signal indicative of the instantaneous velocityof the carriage..].
 13. .[.The improvement set forth in claim 11 inwhich said another sensor comprises.]. .Iadd.In a turning machine (40)comprising means for rotating (48) a part (42) about an axis (44) andfor causing a cutting tool (56) to make an axial pass along the partwhile the radial position of the tool relative to the part isselectively controlled in correlation with the angular position of thepart about the axis of rotation as the part is being rotated, theimprovement which comprises a main CNC control (58), angular positionsensing means (54) operatively coupled with said means for rotating thepart to provide to the CNC control information representative ofinstantaneous angular position of the part about the axis for rotationas the part is being rotated, radial position sensing means (74) forsensing the radial position of the cutting tool relative to the part,radial velocity sensing means (76) for sensing the radial velocity ofthe tool relative to the part, and a closed loop control system (80) forclosed loop controlling the radial position of the cutting tool relativeto the part as a function of the radial position of the cutting tool andthe radial velocity of the cutting tool, said closed loop control systemcomprising an electric linear motor (72) as the prime mover for radiallypositioning the cutting tool, said CNC comprising means for issuingcommands to the closed loop control system correlated with theinformation with said position sensing means and information about thepart, and said closed loop control system comprising means forconverting the CNC commands into a corresponding control current for thelinear motor in which the linear motor is mounted on a head of theturning machine and is operatively connected to a carriage on which thecutting tool is mounted, and said closed loop control system includingat least one sensor on the machine's head operated by movement impartedto said carriage by said linear motor, said at least one sensorproviding a signal for use by the closed loop control system, the atleast one sensor comprising .Iaddend.a fixed reference scale .Iadd.(220)in the form of a grating .Iaddend.mounted on the carriage for movementtherewith and a sensing head .Iadd.(222) .Iaddend.mounted on themachine's head .Iadd.laterally .Iaddend.adjacent the line of travel ofthe scale, the motion of the scale past said sensing head causing saidsensing head to provide the position signal correlating the position ofthe scale, and hence the carriage, on the machine's head.
 14. Theimprovement set forth in claim 13 in which .Iadd.the closed loop controlsystem includes plural sensors, .Iaddend.a further of said pluralsensors senses when said scale is in juxtaposition to said sensing headto thereby define a range over which said sensing head is confined toprovide said position feedback signal.
 15. .[.The improvement set forthin claim 1.]. .Iadd.In a turning machine (40) comprising means forrotating (48) a part (42) about an axis (44) and for causing a cuttingtool (56) to make an axial pass along the part while the radial positionof the tool relative to the part is selectively controlled incorrelation with the angular position of the part about the axis ofrotation as the part is being rotated, the improvement which comprises amain CNC control (58), angular position sensing means (54) operativelycoupled with said means for rotating the part to provide to the CNCcontrol information representative of instantaneous angular position ofthe part about the axis for rotation as the part is being rotated,radial position sensing means (74) for sensing the radial position ofthe cutting tool relative to the part, radial velocity sensing means(76) for sensing the radial velocity of the tool relative to the part,and a closed loop control system (80) for closed loop controlling theradial position of the cutting tool relative to the part as a functionof the radial position of the cutting tool and the radial velocity ofthe cutting tool, said closed loop control system comprising an electriclinear motor (72) as the prime mover for radially positioning thecutting tool, said CNC comprising means for issuing commands to theclosed loop control system correlated with the information with saidposition sensing means and information about the part, and said closedloop control system comprising means for converting the CNC commandsinto a corresponding control current for the linear motor, .Iaddend.inwhich the linear motor is mounted on a head of the turning machine andis operatively connected to a carriage on which the cutting tool ismounted, and said closed loop control system comprising a positioncontrol loop which exercises principal control over said linear motor inpositioning said carriage and also a velocity control loop in whichvelocity feedback information supplements the principal control providedby the position loop control .Iadd.in which the machine comprises meansfor causing said closed loop control system to operate selectively in aposition control mode in which the position and the velocity controlloops are both active and in a velocity control mode in which only thevelocity feedback loop, and not the position feedback loop, isactive.Iaddend.. .[.16. The improvement set forth in claim 15 in whichthe machine comprises means for causing said closed loop control systemto operate selectively in a position control mode in which the positionand the velocity control loops are both active and in a velocity controlmode in which only the velocity feedback loop, and not the positionfeedback loop, is active..].
 17. The improvement set forth in claim.[.16.]. .Iadd.15 .Iaddend.in which the machine comprises means forexecuting the velocity control mode of operation at the beginning of anoperating sequence on a part until the carriage has been advanced from ahome position to a position where the cutting tool is just off the part,and thereafter executing the position control mode of operation toconduct turning operations on the part.
 18. The improvement set forth inclaim 17 in which said closed loop control system comprises a registerwhich is set to a zero reference when operation switches from thevelocity control mode to the position control mode, and including aswitch in the position control loop forward of the velocity control loopwhich is operated concurrent with the setting of said register to thezero reference to thereby render the position loop active.
 19. .[.Theimprovement set forth in claim 1.]. .Iadd.In a turning machine (40)comprising means for rotating (48) a part (42) about an axis (44) andfor causing a cutting tool (56) to make an axial pass along the partwhile the radial position of the tool relative to the part isselectively controlled in correlation with the angular position of thepart about the axis of rotation as the part is being rotated, theimprovement which comprises a main CNC control (58), angular positionsensing means (54) operatively coupled with said means for rotating thepart to provide to the CNC control information representative ofinstantaneous angular position of the part about the axis for rotationas the part is being rotated, radial position sensing means (74) forsensing the radial position of the cutting tool relative to the part,radial velocity sensing means (76) for sensing the radial velocity ofthe tool relative to the part, and a closed loop control system (80) forclosed loop controlling the radial position of the cutting tool relativeto the part as a function of the radial position of the cutting tool andthe radial velocity of the cutting tool, said closed loop control systemcomprising an electric linear motor (72) as the prime mover for radiallypositioning the cutting tool, said CNC comprising means for issuingcommands to the closed loop control system correlated with theinformation with said position sensing means and information about thepart, and said closed loop control system comprising means forconverting the CNC commands into a corresponding control current for thelinear motor, .Iaddend.in which said closed loop control systemcomprises position feedback with means to absolutely relate positionfeedback to the cutting tool position .Iadd.which said closed loopcontrol system further comprises velocity feedback coactive with theposition feedback in which said closed loop control system comprisesmeans for applying the transfer function

    K.sub.v /sT+1

to the velocity feedback to modify same, and the transfer function

    K.sub.p /sT+1

to the position feedback to modify same, and means for summing the twomodified feedback signals and subtracting the sum from a demand positionamplified K_(i) x_(i) at a position summing junction to create aposition error signal for the principal control of the tool positionwherein T is chosen to satisfy the relationship

    T=K.sub.v /K.sub.p

wherein x_(i) is the demand position, K_(i) is gain applied to thedemand position, K_(p) is the gain of the position feedback, K_(v) isthe gain of the velocity feedback, and s is the Laplace operatorsymbol.Iaddend.. .[.20. The improvement set forth in claim 19 in whichsaid closed loop control system further comprises velocity feedbackcoactive with the position feedback..]. .[.21. The improvement set forthin claim 20 in which said closed loop control system comprises means forapplying the transfer function

    K.sub.v /sT+1

to the velocity feedback to modify same, and the transfer function

    K.sub.p /sT+1

to the position feedback to modify same, and means for summing the twomodified feedback signals and subtracting the sum from a demand positionamplified K_(i) x_(i) at a position summing junction to create aposition error signal for the principal control of the tool positionwherein T is chosen to satisfy the relationship

    T=K.sub.v /K.sub.p

wherein x_(i) is the demand position, K_(i) is gain applied to thedemand position, K_(p) is the gain of the position feedback, K_(v) isthe gain of the velocity feedback, and s is the Laplace operatorsymbol..].
 22. The improvement set forth in claim .[.21.]. .Iadd.19.Iaddend.further including a feed-forward branch forward of saidposition summing junction, said feed-forward branch comprising means forproviding compensation to the demand position to anticipate the size offuture change in demand postion.
 23. The improvement set forth in claim22 in which said closed loop control system comprises means to modifythe velocity feedback by gain K_(v2) and to apply the last-mentionedmodified velocity feedback to a further summing junction at which theposition error signal and the last-mentioned modified velocity feedbackare processed to created a modified postion error signal which is usedto control the linear motor, and wherein the following relationshipsexist: ##EQU3## wherein the velocity feedback signal is sx₀.
 24. .[.Theimprovement set forth in claim 19.]. .Iadd.In a turning machine (40)comprising means for rotating (48) a part (42) about an axis (44) andfor causing a cutting tool (56) to make an axial pass along the partwhile the radial position of the tool relative to the part isselectively controlled in correlation with the angular position of thepart about the axis of rotation as the part is being rotated, theimprovement which comprises a main CNC control (58), angular positionsensing means (54) operatively coupled with said means for rotating thepart to provide to the CNC control information representative ofinstantaneous angular position of the part about the axis for rotationas the part is being rotated, radial position sensing means (74) forsensing the radial position of the cutting tool relative to the part,radial velocity sensing means (76) for sensing the radial velocity ofthe tool relative to the part, and a closed loop control system (80) forclosed loop controlling the radial position of the cutting tool relativeto the part as a function of the radial position of the cutting tool andthe radial velocity of the cutting tool, said closed loop control systemcomprising an electric linear motor (72) as the prime mover for radiallypositioning the cutting tool, said CNC comprising means for issuingcommands to the closed loop control system correlated with theinformation with said position sensing means and information about thepart, and said closed loop control system comprising means forconverting the CNC commands into a corresponding control current for thelinear motor in which said closed loop control system comprises positionfeedback with means to absolutely relate position feedback to thecutting tool position, .Iaddend.in which the position feedback isprovided by a fixed reference scale moveable with the tool and a sensinghead past which the scale moves when the tool is operated by the linearmotor, the motion of the scale past the sensing head causing the sensinghead to provide said position feedback, said closed loop control systemfurther comprising an on-scale sensor for confirming when said scale isin juxtaposition to said sensing head to thereby define a range overwhich said sensing head is confirmed to provide said position feedbackand further including a home sensor for sensing a home position of thecutting tool, said machine comprising means for causing said closed loopcontrol system to operate selectively in a position control mode inwhich said position feedback as confirmed by said on-scale sensorprovides position feedback acted upon by the closed loop control systemto exercise principal control over the operation of the linear motor andin a velocity control mode in which the velocity feedback alone is actedupon by the closed loop control system to exercise control overoperation of the linear motor, and means for causing the velocitycontrol mode of operation to occur between the home position defined bythe home sensor and a position defined by a zero reference set in thecontrol relative to an absolute reference set by the position feedback.In a turning machine comprising means for rotating a part about an axisand for causing a cutting tool to make an axial pass along the partwhile the radial position of the tool relative to the part isselectively controlled in correlation with the angular position of thepart about the axis of rotation as the part is being rotated, theimprovement which comprises a main CNC control, position sensing meansoperatively coupled with said means for rotating the part to provide tothe CNC control information representative of instantaneous angularposition of the part about the axis of rotation as the part is beingrotated, radial position sensing means for sensing the radial positionof the cutting tool relative to the part, radial velocity sensing meansfor sensing the radial velocity of the tool relative to the part, and aclosed loop control system for closed loop controlling the radialposition of the cutting tool relative to the part as a function of theradial position of the cutting tool and the radial velocity of thecutting tool, said closed loop control system comprising an electriclinear motor for radially positioning the cutting tool, said CNCcomprising means for issuing commands to the closed loop control systemcorrelated with information from said position sensing means andinformation about the part, and said closed loop control systemcomprising means for converting the CNC commands into a correspondingcontrol signal for the linear motor and wherein said tool is mounted ona carriage which is guided by guide means on a head with the carriagebeing operatively coupled with the linear motor so as to be selectivelypositionable along a line of travel on the head in accordance withoperation of the linear motor by the control signal developed from theCNC commands, said carriage comprising a hollow bar having an exteriorsurface which in transverse cross section is polygonal and in which saidguide means comprises a plural sets of rollers providing rolling contactwith the exterior surface of said hollow bar as said hollow bar isoperated by said linear motor and in which at least one set of rollerscomprises plural non-yieldably mounted rollers and plural yieldablymounted rollers, such yieldably and non-yieldably mounted rollerscoacting with respective exterior surface portions of said hollow bar toprovide for .[.yieldablr.]. .Iadd.yieldably .Iaddend.forceful constraintof the hollow bar while allowing the bar to be rolled thereon.
 26. Theimprovement set forth in claim 25 in which said bar's exterior crosssectional shape is rectangular and there are two sets of rollers,orthogonally related, each set coacting on respective opposite exteriorsurface portions of said bar and in which the non-yieldably mountedrollers of each set comprise a forward pair and a rearward pair havingrolling contact with said bar at space point parts and the yieldablymounted rollers of each set comprise a forward pair and a rearward pairhaving rolling contact with said bar at substantially the same forwardand rearward locations as for the coacting non-yieldably mounted ones.27. The improvement set forth in claim 26 in which the yieldably mountedroller pairs of each set are mounted on the head by means of acorresponding leaf spring mechanism and further including means forselectively positioning at least one pair of each set's non-yieldablymounted rollers toward and away from the axis of the hollow bar.
 28. Theimprovement set forth in claim 26 in which the rollers are disposed onaxles and the rollers roll on margins of said exterior surface portionsimmediately adjacent edges at which said exterior surface portions meet.29. The improvement set forth in claim 25 in which said linear motorcomprises a linearly moveable armature coaxial with the axis of thehollow bar and attached to the hollow bar.
 30. The improvement set forthin claim 29 in which said armature comprises a tubular bobbin having acircular annular transverse cross sectional shape and a cap via whichthe bobbin attaches at an axial end thereof to the hollow bar, and stopson the head disposed for abutment by the cap to define limits of travelfor the carriage and bobbin on the head.
 31. The improvement set forthin claim 29 in which said armature comprises a tubular bobbin having anopen axial end, said open axial end being disposed within a circularannular free space containing a magnetic field, said bobbin comprising acoil thereon disposed within the magnetic field of said free space, saidcoil receiving a control current constituting the control signal for theprime mover with the interaction between the magnetic field of said freespace and the control current in said coil operates the prime mover, andhence the carriage.
 32. The improvement set forth in claim 25 includingplural sensors on said head providing position and velocity informationderived from operation of the carriage by the linear motor for.[.used.]. .Iadd.use .Iaddend.by the closed loop control.
 33. Theimprovement set forth in claim 32 in which one of said sensors is avelocity sensor comprising a coil mounted on the head and a core whichis moveable with the carriage so as to be selectively positionable withrespect to said coil by operation of said linear motor to thereby causea signal to be created in said coil representative of instantaneousvelocity of said carriage and another of said sensors is a positionsensor disposed on the head laterally adjacent the carriage, saidposition sensor comprising a sensing head mounted on the machine's headand a scale attached to the carriage with the scale disposed for travelwith the carriage along a line of travel past the sensing head forcausing the sensing head to provide signals containing the positioninformation.
 34. In a turning machine having a head on which a tool isselectively positionable toward and away from a workpiece by a primemover and a carriage.Iadd., .Iaddend.an improved prime mover andcarriage combination for selectively positioning the tool on the headcharacterized by low friction, low inertia, and rapid response to inputcommands and comprising an electric linear motor forming the prime moverand a bar forming the carriage, said bar having plural exterior surfaceportions via which the bar rolls on coacting sets of rollers on thehead, said coacting sets of rollers being arranged to constrain .[.to.]..Iadd.the .Iaddend.bar to motion along a line of travel toward and awayfrom the part while applying yieldably forceful engagement of the bar,and wherein the bar is further defined to have a rectangular exteriorcross sectional shape so that said plural exterior surface portionscomprise two pairs of parallel surfaces with the two pairs being 90°apart about the axis of the bar, and in which for each pair of parallelsurfaces there is a corresponding coacting set of rollers, each coactingset comprising a set of plural non-yieldably mounted rollers and a setof plural yieldably mounted rollers.
 35. The improved prime mover andcarriage combination set forth in claim 34 in which each set ofyieldably mounted rollers is yieldably mounted on the head by a leafspring mechanism.
 36. The improved prime mover and carriage combinationset forth in claim 35 in which each set of non-yieldably mounted rollersincludes means for selectively positioning at least some of its rollerstoward and away from the axis of the bar.
 37. The improved prime moverand carriage combination set forth in claim 36 in which said means forselectively positioning at least some of the rollers of eachnon-yieldably mounted set toward and away from the axis of the hollowbar comprises a yoke containing such rollers and an adjustment screwmechanism engaging said yoke, said adjustment screw mechanism having athreaded engagement with the head to provide for the selectivepositioning of the yoke by rotating the adjustment screw mechanism andmeans for locking the adjustment screw mechanism against furtherrotation once the desired yoke positioning has been attained.
 38. Theimproved prime mover and carriage combination set forth in claim 37 inwhich said yoke comprises a hole, said adjustment screw mechanism havinga distal end extending into and engaging the bottom of said hole. 39.The improved prime mover and carriage combination set forth in claim 35in which each set of yieldably mounted rollers comprises a forward pairand a rearward pair mounted on their respective yokes, each leaf springmechanism having opposite ends which attach respectively to thecorresponding two yokes, and means for attaching each leaf springmechanism centrally thereof to the head.
 40. The improved prime moverand carriage combination set forth in claim 39 in which each set ofnon-yieldably mounted rollers comprises a rearward pair and at least oneforward pair, each such last-mentioned forward pair and rearward pairbeing disposed on respective yokes one of which is fixedly secured onthe head and the other of which is selectively positionable toward andaway from the axis of the hollow bar and when selectively positioned toa desired position is locked against any further displacement.
 41. Theimproved prime mover and carriage combination set forth in claim 34 inwhich said bar is hollow throughout its entire length, said tool beingmounted at the forward end of said bar, and a cap at the rearward end ofsaid bar via which the bar attaches to the linear motor, said linearmotor being arranged coaxially with said bar.
 42. The improved primemover and carriage combination set forth in claim 41 including stops onthe head disposed for abutment by the cap to define limits of travel forthe carriage on the head.
 43. The improved prime mover and carriagecombination set forth in claim 41 in which said prime mover comprises anarmature in the form of a tubular bobbin which attaches at one axial endthereof to said cap and with the opposite axial end thereof being open,said open axial end being disposed within a circular annular free spacecontaining a magnetic field, and said bobbin comprising a coil thereondisposed within the magnetic field of said free space, said coil beingadapted for receiving control current for interaction with the magneticfield of said free space to operate said bar along its line of travel.44. The improved prime mover and carriage combination set forth in claim43 in which the magnetic field of said free space is provided by atubular magnetic having a circular annular cross sectional shapedisposed concentrically around said free space. .[.45. A method ofcontrolling the radial position of a tool relative to the rotationalaxis of a workpiece in a turning cperation whrein the surface geometryof the workpiece is defined by a data matrix of axial, angular andradial position coordinates, said method comprising the stepsof:generating an anular position signal representative of the currentangular position of the workpiece and an axial position signalrepresentative of the current aaxial position of the tool relative tothe workpiece; correlating the angular and axial position signals withthe data matrix to generate a demand control signal as a function of theangular and axial position signals; generating an actual radial positionsignal representative of the current radial position of the tool and aradial velocity signal representative of the current radial velocity ofthe tool; generating a tool feedback signal having a velocity componentderived from the radial velocity signal and a position component derivedfrom the actual radial position signal; and comparing the demand controlsignal to the tool feedback signal to produce a tool adjustment signalfor controlling the radial position of the tool as both a function toolposition and velocity..]. .[.46. The method of claim 45 wherein thedemand control signal has a first component based on the prior demandedradial position value from the data matrix, and a second component basedon the difference of the current demanded radial position value and theprior demanded radial position value..]. .[.47. The method of claim 46wherein the second component is a feed forward signal which is afunction of the difference in said values..]. .[.48. The method of claim46 wherein the demand control signal is obtained by summing the firstcomponent and the second component..]. .[.49. The method of claim 46wherein the first component is modified by integrating the secondcomponent and summing the integral of the second component with thefirst component..]. .[.50. The method of claim 45 wherein the toolfeedback signal is obtained by summing the velocity component and theposition component..]. .[.51. The method of claim 45 comprising thefurther step of modifying the tool adjustment signal by negativefeedback of the radial velocity signal..]. .[.52. A system forcontrolling the radial position of a tool relative to the rotationalaxis of a workpiece whrein the surface geometry of the workpiece isdefined by a data matrix of axial, angular and radial positioncoordinates, the system comprising: angular position sensor means (54)for producing an angular position signal representative of the currentangular position of the workpiece (42); axial position sensor means (66)for producing an axial position signal representative of the currentaxial position of the tool (56) relative to the workpiece; CNC means(58) for correlating the angular and axial position signals to the datamatrix to produce a demanded radial position signal corresponding to theangular and axial position signals; an electric linear motor coupled tothe tool for radial movement of the tool in accordance with a toolcontrol signal; radial velocity sensor means (76) for producing avelocity signal representative of the radial velocity of the tool;radial position sensor means (74) for producing an actual radialposition signal representative of the current radial position of thetool; and control means (80) for,producing a tool feedback signal havinga velocity component derived from the radial velocity signal and aposition component derived from the actual radial position signal,producing a demand control signal derived from the demanded radialposition signal, and comparing the demand control signal and the toolfeedback signal to produce the tool control signal for controlling theradial position of the tool as a function of both tool position andvelocity..]. .[.53. The system of claim 52 wherein the second controlmeans includes demand signal processing means for producing a demandcontrol signal having a first component (404) based on the priordemanded radial position signal, and a second component (412) based onthe difference of the current demanded radial position signal and theprior demanded radial position signal..]. .[.54. The system of claim 53wherein the demand signal processing means further includes means (420)for operating on the difference signal to produce a feed forward signalwhich is a function of the difference signal..]. .[.55. The system ofclaim 53 wherein the demand signal processing means further includesmeans for summing (422) the first component and the second component toobtain the demand control signal..]. .[.56. The system of claim 53wherein the demand signal processing means includes means forintegrating (418) the second component and summing (408) the integral ofthe second component with the first component to modify the firstcomponent..]. .[.57. The system of claim 52 wherein the control meansincludes tool signal processing means for summing (460) the velocitycomponent and the position component to obtain the tool feedbacksignal..]. .[.58. The system of claim 52 wherein the control meansfurther comprises means for modifying the tool adjustment signal bynegative feedback (466) of the radial velocity signal..]. .[.59. Incombination with a machine tool (40) of the type having a spindle (48)for rotating a workpiece (42) to be machined, a cutting head (56), anelectric linear motor (72) for moving the cutting head (56) radiallyrelative to the spindle axis (44), control means for producing an outputsignal for controlling the linear motor to machine the workpiece to havea configuration that is defined by a stored part program, said controlmeans including means (54) for sensing the angular position of saidspindle, means (66) for sensing the axial position of said cutting headand means (74) for sensing the radial position of said cutting head, theimprovement characterized in that:said control means includes means (76)for sensing the radial velocity of said cutting head (56) and producessaid output signal for controlling said linear motor in response to saidangular position sensing means, said axial position sensing means, saidradial position sensing means and said radial velocity sensing means..]..[.60. The machine tool of claim 59 wherein the closed loop controlsystem performs the step of comparing a demand control signal which is afunction for the programmed position of the cutting head to a feedbacksignal which is a function of at least the radial position of thecutting head..]. .[.61. The machine tool of claim 60 wherein the outputsignal for controlling the linear motor is a function of the comparedsignals and the radial velocity of the cutting head..]. .[.62. Themachine tool of claim 60 wherein the feedback signal is also a functionof the radial velocity of the cutting head..]. .[.63. The machine toolof claim 62 wherein the closed loop control system develops the feedbacksignal by a summation (422) of signals based on the radial position andthe radial velocity of the cutting head..]. .[.64. The machine tool ofclaim 59 further comprising CNC means (58), cooperative with the closedloop control system, for generating a prgrammed radial position signalbased on a correlation of the angular position of the spindle and theaxial position of the cutting head, for use in deriving the demandcontrol signal..]. .[.65. The machine tool of claim 63 wherein theclosed loop control system further includes feed forward signalprocessing means for producing a demand control signal which is afunction of the present programmed radial position signal and the nextsuccessive programmed radial position signal..]. .[.66. An improvedmachine tool (40) of the type having a spindle (48) for rotating aworkpiece (42) to be machined, a cutting head (56), an electric linearmotor (72) for moving the cutting head radially relative to the spindleaxis (44) to machine the workpiece to have configuration that is definedby a stored part program, said linear motor being controlled by closedloop control system (80) responsive to the radial position of thecutting head as derived from radial position sensing means (74) and theradial velocity of the cutting head as derived from radial velocitysending means (76), the improvement characterized in that: the closedloop control system provides an output signal for controlling the linearmotor which is a function of the present and the next successiveprogrammed positions of the cutting head..]. .[.67. The machine tool ofclaim 66 wherein the closed loop control system develops a feed forwardsignal which is a function of the difference of the present and the nextsuccessive programmed positions of the cutting head..]. .[.68. Themachine tool of claim 67 wherein the feed forward signal is combinedwith a signal representative of the present programmed position todevelop a demand control signal..]. .[.69. The machine tool of claim 68wherein the output signal for controlling the linear motor is a functionof the demand control signal, the radial position of the cutting headand the radial velocity of the cutting head..]. .[.70. In combinationwith a machine tool (40) of the type including a spindle (48) forrotating a workpiece (42) to be machined about a spindle axis (44), acutting head (56), an electric linear motor (72) responsive to a controlsignal for moving the cutting head (56) radially relative to saidspindle axis (44), and control means (80, 58, 54, 66, 74) for providinga control signal to the electric linear motor to machine the workpieceto have a configuration defined by a stored part program, said controlmeans including angular position sensing means (54) for sensing theangular position of the workpiece (42) about the spindle axis (44),axial position sensing means (66) for sensing the axial position of thecutting head (56) along the spindle axis (44), and radial positionsensing means (74) for sensing the radial position of said cutting head(56) relative to said spindle axis (44) to develop the control signal asa function of the sensed angular, axial and radial positions, theimprovement characterized in that: the control means further comprisesradial velocity sensing means (76) to sense the radial velocity of thecutting head (56) relative to said spindle axis (44) to develop animproved control signal as a function of the sensed angular, axial andradial positions and the sensed radial velocity..]. .[.71. The inventionof claim 70 wherein the control means develops the control signal from acomparison of a demand control signal derived from the sensed angularand axial positions to a feedback signal derived from the sensed radialposition and the sensed radial velocity..]. .[.72. The invention ofclaim 71 wherein the control means includes CNC means (58) forcorrelating the sensed angular and axial positions to a programmedposition which is functionally related to the demand control signal..]..[.73. The invention of claim 71 wherein the control means derives thefeedback signal from a summation (422) of signals based on the sensedradial position and the sensed radial velocity..]. .[.74. The inventionof claim 71 wherein the control means further includes feed forwardsignal processing means for producing a demand control signal which is afunction of the present programmed position and the next successiveprogrammed position..]. .Iadd.75. In a turning machine having a head onwhich a tool is selectively positionable toward and away from aworkpiece by a prime mover and a carriage, an improved prime mover andcarriage combination for selectively positioning the tool on the headcharacterized by low friction, low inertia, and rapid response to inputcommands and comprising an electric linear motor forming the prime moverwherein one end of the carriage is adapted to be coupled to the cuttingtool, the other end of the carriage is coupled to the motor fortransmitting linear motion and wherein said linear motor and carriageare mounted on the head of the machine, and in which said carriage isguided for reciprocable linear motion on the machine head by guide means(138,178) disposed in cooperative relation with the carriage and themachine head for facilitating sliding movement of the carriage, andspring means (148) for spring-loading the guide means with a springforce sufficient to resist displacement in any direction transverse tothe radial direction and thereby minimize deflection of the carriage inany such direction. .Iaddend. .Iadd.76. The combination of claim 75further comprising means (166,182,184) for precisely locating the toolcarriage relative to the tool head at a predetermined referenceposition. .Iaddend. .Iadd.77. The combination of claim 75 wherein thecarriage (128) is hollow. .Iaddend. .Iadd.78. The combination of claim75 or claim 77 wherein the carriage (128) has an exterior surface whichin transverse cross section is polygonal. .Iaddend. .Iadd.79. Thecombination of claim 75 wherein the guide means includes plural sets ofrollers. .Iaddend. .Iadd.80. The combination of claim 79 wherein atleast one set of rollers includes plural non-yieldably mounted rollersand plural yieldably mounted rollers coacting on respective portions ofthe carriage (128). .Iaddend. .Iadd.81. The combination of claim 80wherein the yieldably mounted rollers are pre-loaded by said springmeans. .Iaddend. .Iadd.82. In a turning machine having a head on which atool is selectively positionable toward and away from a workpiece by aprime mover and a carriage an improved prime mover and carriagecombination for selectively positioning the tool on the headcharacterized by low friction, low inertia, and rapid response to inputcommands and comprising an electric linear motor forming the prime moverwherein one end of the carriage is adapted to be coupled to the cuttingtool, the other end of the carriage is coupled to the motor fortransmitting linear motion and wherein said linear motor and carriageare mounted on the head of the machine; and a fixed reference scale(220) in the form of a grating mounted on the carriage for movementtherewith and a sensing head (222) mounted on the machine head laterallyadjacent the line of travel of the scale (220), the motion of the scale(220) past said sensing head (222) causing said sensing head (222) toprovide a position signal correlating the position of the scale (220),and hence the carriage on the machine head. .Iaddend.